The present invention relates to a circuit that controls the brightness of a light. More specifically, the present invention pertains to a circuit that controls the brightness of a light connected to a dimmable electronic ballast such as the Mark VII Fluorescent Lamp Ballast manufactured by Advance Transformer Company.
The Mark VII Fluorescent Lamp Ballast provides a pair of output leads through which it supplies a DC current loop of between 300 and 500 microamps. To control the intensity level of a light connected to the Mark VII ballast, the voltage level between these two leads is adjusted between 2 and 10 volts. At 2 volts, the light connected to the ballast is at its minimum output of 20 percent. While at 10 volts, the light is operating at the 100 percent level.
It is desirable: to control the brightness of lights connected to the Mark VII ballast in response to the level of ambient light in an area or room. When the ambient light level is low, the lights can be operated at their 100 percent output level to provide maximum lighting for the room, and when the ambient light level is high, the Mark VII ballast can dim the output of the lights to save electricity.
A known prior art circuit manufactured by Multipoint Lighting Control Systems controls the brightness of lights connected to the Mark VII ballast using a reversed-biased photo sensor to detect the ambient light level in a room. Reverse-biasing a photo sensor, however, results in a nonlinear response to the detected light level. Thus, the Multipoint circuit cannot control the Mark VII ballast in a manner such that the ballast accurately maintains a constant light level in a room.
Additionally, in determining the brightness level of the light connected to the ballast, the Multipoint circuit compares the output of the photo detector after it has been amplified by a transistor to a reference voltage created by the voltage drop across a base and emitter of a transistor that can vary significantly with the temperature. Using an unstable voltage as a reference voltage also detracts from the control circuits ability to accurately maintain a constant light level in a room.